That is a big question. To name a few things Fullmetal Alchemist has done for me, I will make a list here. To name a few:
-Inspired what I want my career to be when I'm older (scientist or manga artist)
-I learned what alchemy was
-I learned what a chimera was
-I learned about the philosiphers stone was
-I learned what homunculi are
-Inspired ideas for if I'm a manga artist or if I'm a scientist (I will mention one example of this: I have started writing an action manga)
-To appreciate that I have all my limbs/an actual body
-Lots of military terms
-Politics
-What the seven sins were (I had heard of "The seven deadly sins" before, but never knew what they were)
-The makeup of an adult human body
-Lots of science-related things
-I was inspired to watch lots more anime to find one as good as FMAB (haven't found one yet)
There are too many more to list.
PS Before watching FMAB, the only anime I had watched were Rurouni Kenshin and Fruits Basket. I was told I should watch FMAB, and it has totally changed the course of my life. Without it, I wouldn't be the person I am today. I thank Hiromu Arakawa, author of FMA